Numerous guidance laws have been developed for guiding missiles that use liquid-fuel divert motors during the missile's intercept phase of flight. Unfortunately, liquid fuel is quite toxic and is therefore unsafe in many stowage, handling and operation applications. Solid fuel, on the other hand, is relatively safe. The primary disadvantage of solid-fuel divert motors is that each burn imparts a thrust of a predetermined magnitude. Unlike a liquid fuel, once a solid fuel is ignited, the magnitude and duration of the burn is difficult to alter. This poses a significant constraint on the capability of the vehicle and the design of the guidance law. One guidance method specifically designed for solid-fuel divert motors is addressed in applicant's patent entitled "Method of Guiding an In-Flight Vehicle Toward a Target", U.S. Pat. No. 5,082,200, issued Jan. 21, 1992, the disclosure of which is hereby incorporated by reference. However, such approach is restricted to vehicles with predefined, discrete solid fuel burns (i.e., diverts) separated by coasting periods. Construction of vehicles with such predefined flight scenarios is complex and therefore costly.
Further, in certain types of intercept scenarios, it is necessary for a kill vehicle (i.e., the payload of a missile, commonly referred to as a KV, that actually intercepts a missile or satellite) to accurately know target range during homing. Target range may be necessary for fuzing and/or for the computation of the gains used in the KV's guidance techniques.
Some conventional KVs explicitly measure range with laser or radar range-finding sensors. This solution, however, has drawbacks. It requires an extra device to be installed on the KV, which increases the KV's size, weight, complexity and power requirements. In addition, the range-finding device may not work in high-velocity space intercepts. In a highvelocity intercept (e.g., relative velocity greater than 2 kilometers per second), the target range is relatively large (e.g., tens of kilometers) until seconds prior to the intercept. Since active range-finding sensors ping a signal off the target, at long range the return signal may be too weak to receive and process. Consequently, the active rangefinding device may be inoperable during most of homing.
Fortunately, range can be determined without direct measurement. If the KV uses an optical sensor to explicitly measure the direction to the target, target range can be determined geometrically via triangulation. To do this, the KV intentionally flies an offset trajectory--a trajectory that, if projected through time, would miss the target. By flying this offset trajectory, the relative geometry of the KV and the target changes over time--in a sense, giving the KV a stereoscopic image of the target. As the KV approaches the target, divert motors (rockets) or aerosurfaces turn the KV onto a collision course.
Conventional guidance techniques, such as proportional navigation, do not direct the KV along an offset trajectory as they try to achieve a collision course as soon as possible. Consequently, these guidance techniques are not suitable for passive range estimation (passive because an optical sensor does not send an active signal that pings off the target). For passive range estimation, the shape of the offset trajectory is important. The simplest offset trajectory is an arc lying in a single plane. In this trajectory, the KV is initially offset from the intercept trajectory. As the KV approaches the target, the magnitude of this offset decreases monotonically to zero. By adding a zig-zag or spiral to this trajectory, the target range becomes more observable.
In the prior art, two guidance techniques have been proposed for passive range estimation. The first of these is known as the "maximum-information" guidance technique disclosed by D. G. Hull and J. L. Speyer in "Maximum-Information Guidance for Homing Missiles," AIAA Journal of Guidance Control and Dynamics, Vol. 8, No. 4, July-August 1985, pp. 494-497. The second of these techniques is known as the dithered, proportional-navigation guidance technique disclosed by David V. Stallard in "An Angle-Only Tracking Filter for a Maneuvering Target," Conference Proceedings of AIAA Guidance, Navigation and Control Conference, 1990.
The maximum-information guidance technique directs the KV in a 2-dimensional zig-zag trajectory to the target. In other words, if the intercept is observed in a Cartesian frame centered at the target, the KV follows a zig-zag flight path that lies in a single plane. To compute this trajectory, the KV must solve an optimization problem requiring computational resources not typically available from a conventional KV guidance computer. Thus, the computational requirements of the maximum-information guidance technique make its realization unlikely for current missile technology. Furthermore, to keep the optimization problem manageable, the intercept trajectory is constrained to lie in a 2-dimensional plane thereby excluding other, possibly more optimal, 3-dimensional trajectories.
The dithered, proportional-navigation guidance technique directs the KV in a 3-dimensional spiral trajectory. This guidance technique simply rotates the KV thrust about the line-of-sight axis at a fixed, predetermined angular rate. Superimposed on this thrust is the thrust commanded by the conventional proportional-navigation guidance technique. Since the thrust requirements of the proportional-navigation guidance technique are not known in advance, the KV must have enough thrust to cover any possible scenario. Extra thrust, i.e., thrust capability not required in a given scenario, is throttled. While this may be acceptable for KVs equipped with liquid-fuel divert motors, a KV equipped with an unthrottled solid fuel divert motor cannot be guided in accordance with the dithered, proportional-navigation guidance technique.